Pearl luster pigments consist typically of 200 to 1000 nm thick platelet-shaped substrates coated with 50 to 300 nm thick, highly light-refractive metal oxides or metal oxide mixtures in different oxidation states. The optical properties of these pigments are decisively determined by the refractive index of the metal oxide layer. In contradistinction to metal oxide layers produced by chemical vapor deposition (CVD) or physical vapor deposition (PVD), which have high densities and hence refractive indices which are close to the optimum, the deposition of metal oxides onto platelet-shaped substrates is effected by titration of aqueous, usually acidic, metal salt solutions with sodium hydroxide solution in the presence of a substrate, as described for example in DE 14 67 468 and DE 20 09 566.
The character of this aqueous precipitation process does not make it possible to obtain similarly dense layers as in the CVD or PVD processes, and thus the resulting layers exhibit in some instances appreciable porosity. The density and hence the refractive index of such porous metal oxide layers is therefore distinctly below the maximum values which can be achieved. The ability of these pigments to reflect incident light decreases and the color strength and brilliance also decrease. Also, cracks in such metal oxide layers lead to the scattering of light, resulting in reduced transparency.
It is known that boron-containing admixtures activate sintering processes in Fe.sub.2 O.sub.3 crystals and effect the formation of crack-free polycrystalline Fe.sub.2 O.sub.3 films.
SU 16 999 930 A1 discloses red pigments of enhanced pearl luster which are based on iron oxide-coated mica platelets aftercoated with B.sub.2 O.sub.3. The boron-containing iron oxide pigments are calcined at temperatures between 600.degree. and 900.degree. C. in the presence of air to enhance the luster and lightness.
DE 41 35 742 A1 describes B.sub.2 O.sub.3 -doped iron oxide pigments with a particularly smooth surface. The magnetite pigments, produced by the nitrobenzene process, can be converted into red pigments of various tints by subsequent calcination in air.